1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine of an automobile and a process for preparing the catalyst and, more particularly, an exhaust gas purifying catalyst for removing effectively a high concentration hydrocarbons exhausted in engine start-up, and a process for preparing the catalyst.
2. Description of the Related Art
In the prior art, for the purpose of purifying the exhaust gases from the internal combustion engine of the automobile, etc., a three-way catalyst has been employed. The three-way catalyst performs oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) simultaneously.
In Japanese Laid-Open Patent publication Sho 58-20307 published in 1983, a three-way catalyst comprises an alumina coating layer formed on a refractory support and the layer carries noble metals such as palladium (Pd), platinum (Pt), rhodium (Rh), etc. Also another three-way catalyst is added a base metal oxide as an assistant catalyst component.
These three-way catalysts are strongly affected by the exhaust gas temperature and the air-fuel ratio (A/F). In order to make these three-way catalysts fulfill their purifying function effectively, the air-fuel ratio must be set in the vicinity of a stoichiometric air-fuel ratio (A/F=14.6) at which the oxidation of HC and CO and the reduction of NOx are balanced. Also, in order to make the three-way catalysts active, the exhaust gas temperature must be kept at 300xc2x0 C. or more.
Therefore, in the prior art, when the three-way catalyst is fitted to a body of an automobile to purify exhaust gases, such three-way catalyst is positioned to receive the exhaust gases with the proper temperature. Also, feedback control is carried out to hold the mixed gas in the neighborhood of the stoichiometric air-fuel ratio by detecting the oxygen content in the exhaust system.
However, even if the three-way catalyst is positioned directly after an exhaust manifold of the engine, the temperature of the exhaust gases does not come up to 300xc2x0 C. immediately after the engine start-up. A period in which the temperature of the exhaust gases is low in the engine start-up is called a xe2x80x9ccold zonexe2x80x9d. In this cold zone, since a large quantity of HC is exhausted from the engine but the three-way catalyst is not active, such HC is not purified and thus exhausted as it is. Such HC generated in the cold zone is called a xe2x80x9ccold HCxe2x80x9d.
Japanese Laid-Open Patent publications Hei 2-135126 published in 1990and Hei 3-141816 published in 1991, disclose an exhaust gas purifying system for overcoming such xe2x80x9ccold HCxe2x80x9d problem. In this purifying system, an HC trapper in which an adsorbent is filled to adsorb HC being exhausted in the cold zone is arranged at the upstream of the flow of the exhaust gases before the three-way catalyst.
However, in the above exhaust gas purifying system equipped with the HC trapper, before the three-way catalyst arranged at the downstream side of the flow rather than the adsorbent can reach the active temperature, HC is desorbed from the HC trapper arranged at the upstream side of the flow. Therefore, the HC trapper cannot satisfactorily perform its HC trapping function. Further, since zeolite used as the adsorbent is dipped in a metal-based catalyst solution in a process of preparing the exhaust gas purifying system, the durability of the exhaust gas purifying system is insufficient.
Also, like the exhaust gas purifying system set forth in Japanese Laid-Open Patent publication Hei 3-141816 published in 1991, if desorption control of adsorbed HC is carried out by using a temperature sensor, a bypass pipe, a control unit, etc., the system becomes complicated in configuration to be lower reliability and thus an exhaust layout is not practical.
In view of these problems in the prior art, it is an object of the present invention to provide a more practical exhaust gas purifying catalyst capable of purifying effectively a high concentration HC being exhausted in the cold zone immediately after the engine start-up, and a process for preparing the catalyst.
In order to achieve the above object, according to an aspect of the exhaust gas purifying catalyst of the present invention, such exhaust gas purifying catalyst comprises a hydrocarbon adsorbent layer formed on a monolithic support to contain zeolite, and a metal-based catalyst layer formed on the hydrocarbon adsorbent layer to contain at least one type noble metal selected from the group consisting of palladium, platinum, and rhodium. Further, both layers contain an alkaline metal and/or an alkaline earth metal, and a weight ratio of the alkaline metal and/or the alkaline earth metal contained in the metal-based catalyst layer to the alkaline metal and/or the alkaline earth metal contained in the hydrocarbon adsorbent layer is set to 60:40 to 99:1.
According to another aspect of the exhaust gas purifying catalyst of the present invention, since such exhaust gas purifying catalyst of the present invention comprises the metal-based catalyst layer as the overlying layer and the hydrocarbon adsorbent layer as the underlying layer, it can have the function as the HC trapper and the function as the three way catalyst together, and can suppress desorption of HC trapped by the hydrocarbon adsorbent due to the presence of the metal-based catalyst layer to thus achieve the excellent HC trapping function.
Also, since the metal-based catalyst layer contains a sufficient amount of a alkaline metal and/or alkaline earth metal, it prevents degradation of the absorbing performance of the noble metals and improves the durability of the catalyst. In addition, since an amount of alkaline metal and/or alkaline earth metal in the hydrocarbon adsorbent layer is restricted, pores of the zeolite constituting the hydrocarbon adsorbent layer are not filled with the excessive alkaline metal and/or alkaline earth metal. Therefore the hydrocarbon adsorbent layer performs the high HC absorbing ability.
Meanwhile, when the exhaust gases containing O2, HC, NOx flow into the catalyst of the present invention at the low temperature immediately after the engine start-up, etc., HC is adsorbed by the underlying hydrocarbon adsorbent layer and thus the O2 partial pressure in the exhaust gases is increased. In this case, since the NOx adsorbing performance is in proportion to the O2 partial pressure, adsorption of Nox is promoted in the overlying metal-based catalyst layer at this time. Accordingly, the cold HC and the unpurified Nox can be adsorbed at the same time by the catalyst of the present invention. After this, as the activation of the overlying metal-based catalyst layer having a three way purifying function is accelerated when the temperature of the exhaust gases becomes high, HC desorbs from the underlying hydrocarbon adsorbent layer and reacts with NOx adsorbed in the metal-based catalyst layer to be canceled with each other. As a result, the effective exhaust gas purification can be achieved.
According to an aspect of the process for preparing the exhaust gas purifying catalyst of the present invention, in order to prepare the catalyst of the present invention having above features, the metal-based catalyst layer is formed on the hydrocarbon adsorbent layer containing zeolite, by coating the slurry which contains at least one noble metal salt selected from the group consisting of palladium, platinum, and rhodium, and water insoluble or water hardly-soluble alkaline metal and/or alkaline earth metal salt.
Since the alkaline metal and/or the alkaline earth metal is mixed into the slurry in the form of water insoluble or water hardly-soluble alkaline metal and/or alkaline earth metal compound, dissolving-out of the alkaline metal and/or the alkaline earth metal into the slurry be limited. Therefore, diffusion of the alkaline metal into the zeolite as the hydrocarbon adsorbent layer having the strong water absorbing property is suppressed.